LTC3703 Linear Technology, LTC3703 Datasheet - Page 14

• Manufacturer Part Number: LTC3703
• Description: 100V Synchronous Switching Regulator Controller
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
LTC3703
The curve is generated by forcing a constant input current
into the gate of a common source, current source loaded
stage and then plotting the gate voltage versus time. The
initial slope is the effect of the gate-to-source and the gate-
to-drain capacitance. The flat portion of the curve is the
result of the Miller multiplication effect of the drain-to-gate
capacitance as the drain drops the voltage across the
current source load. The upper sloping line is due to the
drain-to-gate accumulation capacitance and the gate-to-
source capacitance. The Miller charge (the increase in
coulombs on the horizontal axis from a to b while the curve
is flat) is specified for a given V
adjusted for different V
ratio of the application V
values. A way to estimate the C
change in gate charge from points a and b on a manufac-
turers data sheet and divide by the stated V
specified. C
for determining the transition loss term in the top MOSFET
but is not directly specified on MOSFET data sheets. C
and C
parameters are not included.
When the controller is operating in continuous mode the
duty cycles for the top and bottom MOSFETs are given by:
14
appropriate breakdown specification. Since many high
voltage MOSFETs have higher threshold voltages (typi-
cally, V
with a 9V to 15V gate drive supply (DRV
For maximum efficiency, on-resistance R
capacitance should be minimized. Low R
conduction losses and low input capacitance minimizes
transition losses. MOSFET input capacitance is a combi-
nation of several components but can be taken from the
typical “gate charge” curve included on most data sheets
(Figure 9).
V
GS
OS
GS(MIN)
are specified sometimes but definitions of these
C
MILLER
MILLER
a
Figure 9. Gate Charge Characteristic
MILLER EFFECT
= (Q
Q
6V), the LTC3703 is designed to be used
IN
B
is the most important selection criteria
U
– Q
A
)/V
b
DS
DS
U
DS
voltages by multiplying by the
to the curve specified V
DS
MILLER
drain voltage, but can be
W
V
+
GS
–
term is to take the
V
DS(ON)
CC
DS(ON)
pin).
+
–
DS
U
minimizes
V
and input
3703 F09
DS
V
voltage
IN
RSS
DS
The power dissipation for the main and synchronous
MOSFETs at maximum output current are given by:
where is the temperature dependency of R
the effective top driver resistance (approximately 2 at
V
in drain potential in the particular application. V
data sheet specified typical gate threshold voltage speci-
fied in the power MOSFET data sheet at the specified drain
current. C
gate charge curve from the MOSFET data sheet and the
technique described above.
Both MOSFETs have I
equation includes an additional term for transition losses,
which peak at the highest input voltage. For V
high current efficiency generally improves with larger
MOSFETs, while for V
rapidly increase to the point that the use of a higher
R
efficiency. The synchronous MOSFET losses are greatest
at high input voltage when the top switch duty factor is low
or during a short circuit when the synchronous switch is
on close to 100% of the period.
The term (1 + ) is generally given for a MOSFET in the
form of a normalized R
typically varies from 0.005/ C to 0.01/ C depending on
the particular MOSFET used.
GS
DS(ON)
P
P
MainSwitchDutyCycle
SynchronousSwitchDutyCycle
MAIN
SYNC
= V
MILLER
device with lower C
MILLER
2
1
D
V
V
MAX
1
CC
), V
OUT
–
D
–
is the calculated capacitance using the
1
IN
2
MAX
1
V
1
TH IL
is the drain potential and the change
–
2
1
I
MAX
R losses while the topside N-channel
D
–
( )
I
DS(ON)
MAX
IN
MAX
(
D
I
MAX
MAX
MILLER
> 25V, the transition losses
V
V
) (
2
TH IL
2
vs temperature curve, and
V
OUT
(
1
IN
1
(
1
R
( )
actually provides higher
DR
)
)(
( )
)
R
f
R
V
C
DR ON
DS N
IN
MILLER
(
( )
–
V
0
IN
DS(ON)
V
)
IN
OUT
TH(IL)
< 25V, the
)•
, R
is the
DR
3703f
is